The two forms encroach but little on each other's territory, but where they do coexist, a third form, known as _proserpina_, is found which is almost intermediate, with the white bands much reduced. There is now no doubt that this _proserpina_ is a heterozygous form, resulting from a combination of the characters of _arthemis_ and _astyanax_. Field succeeded in rearing a brood of 16 from a _proserpina_ mother caught wild which laid 31 eggs, and of these, nine (five males, four females) resembled the mother, being _proserpina_, and seven (four males, three females) were _arthemis_. There can be no question therefore that the mother had been fertilised by a male _arthemis_ and that _no-white-band_ is a factor partially dominant over the _white band_. Another point of interest which Field observed was that the _proserpina_ female refused to lay on birch, poplar or willow, but accepted wild cherry (_Prunus serotina_) a species on which _astyanax_ can live, though that tree is not known to be eaten by _arthemis_. Incidentally also the observations show that sterility cannot be supposed to be the bar which maintains the distinctness of _arthemis_ and _astyanax_.
In this connection _Papilio oregonia_ and _bairdii_ should be mentioned.[14] _P. oregonia_ is one of the numerous forms like _machaon_, but rather paler. It is a northern insect, inhabiting British Colombia east of the Cascade Range, and reaching to Colorado. _P. bairdii_ is a much darker butterfly, representing the _asterias_ group of the genus _Papilio_. Like _asterias_ it has the abdomen spotted at the sides, not banded as in the _machaon_ group. It belongs to Arizona and Utah extending into Colorado. From Colorado the form _brucei_ is described, more or less intermediate, like _bairdii_ but with the abdomen banded as in _oregonia_. W. H. Edwards records the results of rearing the offspring of the _bairdii_-like and of the _oregonia_-like mothers. Each was found able to have offspring of both kinds, that is to say, _bairdii_ females gave both forms, and _oregonia_ females gave both forms. It is not possible to say which is dominant, since the fathers were unknown. On general grounds one may expect that the _bairdii_ form will be found to dominate, but this is quite doubtful.